The invention relates to exhaust aftertreatment filters for filtering exhaust from internal combustion engines, including diesel engines.
Exhaust aftertreatment filters for diesel engines are known in the prior art. The filter traps contaminant particulate present in exhaust, and in order to remove the trapped particulate, the filter is heated to burn-off the trapped contaminant particulate as gas. Accordingly, the filter is regenerable and is composed of material on which trapped contaminant particulate from the engine exhaust is removed by addition of heat. Commonly used particulate filter materials include cordierite, silicon carbide, mullite, or aluminum titanate, which are manufactured as filter elements to capture the soot and other particulate generated by the engine.
Diesel particulate filters (DPF) are subject to high temperatures during use. The design of the DPF consists of a honeycomb structure with opposing channels blocked to force exhaust gases to flow through the porous channel walls, while trapping soot. The soot (composed primarily of carbon) accumulates in the DPF and must be removed periodically. Typically, the soot is removed from the filter by oxidation reactions between carbon in the soot and either oxygen (i.e., burning) or nitrogen dioxide, both of which are constituents of the exhaust. The carbon may react with oxygen or nitrogen dioxide according to the following reactions:C(s)+O2(g)→CO2(g)  (1)C(s)+2NO2→CO2(g)+2NO(g)  (2)Reaction (1) is the primary reaction that occurs during an active regeneration. Reaction (2) is the primary reaction that occurs during passive regeneration. Heat is a significant by-product of the reaction shown in Reaction (1) and, if not controlled, can cause thermal runaway of the filter, leading to fractures and/or melting of the filter and rendering it ineffective as a filter.
Although thermal runaway may be prevented by controlling the rate at which carbon is burned in the filter, nonetheless the DPF may be subject to thermal gradients caused by differential heating patterns, which also may lead to fractures. Differential heating may occur during active regeneration of the filter where carbon may be unequally distributed, either radially or axially, within the DPF. For example, carbon may be more highly distributed in the DPF at locations where the largest volume of exhaust passes through the filter (i.e., at locations where exhaust velocity is highest). These locations may exhibit a relatively high temperature during active regeneration as compared to other locations.
Although carbon distribution may be altered by modifying the design of a filter, particulate filter manufacturers are hampered by material strength issues that limit the maximum porosity that can be obtained in an extruded honeycomb structure. Filter manufacturers typically design filters that have the lowest back pressure and suitable filtration efficiency as required for a particular engine. However, ceramic filter manufacturers have difficulty designing filters that have the lowest back pressure and suitable filtration efficiency without greatly weakening the honeycomb structures. Furthermore, it is commonly believed that filtration efficiency drops precipitously with even a single unblocked or broken channel in the honeycomb structure of the filter, even though unblocked channels might lower back pressure.
Therefore, there is a need for filters having modified design characteristics in order to minimize back pressure and thermal gradients during regeneration. Furthermore, it is desirable that these modified design characteristics can be combined with control techniques to create filters that are more durable and resistant to structural damage which may occur during use of the filter (e.g., during regeneration).